Dual population of GABAA and GABAB receptors in rat pars intermedia demonstrated by release of αMSH caused by barium ions

We have studied the effects of selective GABAA and GABAB agonists on alpha-melanophore stimulating hormone (alpha MSH) release from intact rat neurointermediate lobes (NIL) in vitro. Agonist effects were tested against either basal alpha MSH output or BaCl2 (5 mM)-evoked release. GABA (50 microM) produced a biphasic effect on basal release, with an enhancement followed by inhibition of release. The enhancement but not the inhibition was blocked by bicuculline methiodide (100 microM). Baclofen (10 microM), a specific GABAB agonist, reduced the basal and Ba2+-evoked hormonal release in a stereospecific manner. (-)-Baclofen (5 microM) was active whereas the (+)-isomer was inactive at the same concentration. Isoguvacine (50 microM) a specific GABAA agonist, potentiated the Ba2+-evoked release of alpha MSH. GABA (50 microM) mimicked this effect, and its action was antagonized by bicuculline methiodide (200 microM). The results suggest that both GABAA and GABAB receptors are present on the endocrine cells of the intermediate lobe.